<!doctype html>
<html>
<head>
<meta charset="utf-8">
<script>
document.addEventListener("DOMContentLoaded", () => {
  const input = document.querySelector("input");
  input.focus();
  input.type = "text";
  input.value = "new value";
}, {once: true});
</script>
</head>
<body><input type="button" contenteditable></body>
</html>
